1. Field of the Invention
This invention relates to an apparatus and method for determining the surface topography of the cornea and sclera of an eye.
2. Brief Description of the Prior Art
Methods and instruments for determining the surface curvature of the cornea by keratometry are known. Keratometers utilize a variety of optical systems to observe the size and shape of the image of a target mire as reflected and refracted from the eye. Information regarding the surface symmetry and curvature of the cornea in a relatively small area near the center of the cornea may be deduced using keratometers, such as recently disclosed in U.S. Pat. Nos. 4,420,228 or 4,429,960. However, because all keratometry involves the approximation of the corneal surface as one or another relatively high symmetry conic surface, such as a spherical or ellipsoidal surface, information obtained by keratometry about the actual corneal topography is limited. The problem is acute when attempting to examine pathological and post-surgical corneas. The information obtained by keratometry is often insufficiently detailed for the proper fitting of contact lenses, or for the demands of eye surgery. The utility of keratometry is further limited in that it can be used to measure eye surface curvature only in the center of the cornea and not in the periphery of the cornea, in the transition area between the cornea and the sclera (the limbus), or on the sclera itself.
Several techniques have been proposed for measuring the topography of the corneal surface. These include stereophotography of a light pattern projected onto the eye and analysis of the stereophotographs photogrammetrically (Friedberg, et al., U.S. Pat. No. 3,169,459). Derivatives of the surface curvature have been measured both manually (Volk, U.S. Pat. No. 3,486,812) and by photography of reflected light through a telecentric optical system, followed by integration of the data to determine the topography (Fujii, et al., 19 Optica Acta 425 (1972)). Kawara, 18 Applied Optics 3675 (1979), discloses a method of determining topography by photographing moire contour fringes derived from a projected fluorescence pattern. Determination of the corneal profile has been accomplished by illumination of the eye with a slit lamp in combination with recording by photography (Kimball, et al., U.S. Pat. Nos. 3,141,396 and Papritz, 3,519,338) or a vidicon camera (Cornsweet, et al., U.S. Pat. No. 4,019,813). The phase shifts of the reflections of sonic compressional waves have also been measured to determine eye topography (Friedberg, U.S. Pat. No. 3,442,579). None of these approaches to topography determination has found wide clinical acceptance due to excessive cost, lack of accuracy or inconvenience of use. Some of these techniques are restricted to measurements of just the corneal surface. Photographic procedures involve two distinct steps; after the photographs are taken, they must be separately analyzed. Further, such procedures are prone to calibration and measurement errors due to film dynamics and uncertainties in the measurements of indistinct images. Such procedures are also time consuming and inconvenient in the clinical setting. Computer analysis of vidicon pictures tends to be expensive and is often inaccurate due to signal extraction problems.